The invention relates to a device for the linear actuation of a control member, particularly a valve tappet in a gas or liquid conduit, the device comprising a rotor having an inner screwthread whose rotary movement is convertible into a linear movement of a threaded spindle which is locked against rotation and which is engageable with the inner screwthread.
Such a device is known from DE 44 23 313 A1. This known device comprises a double-strand claw pole stepper motor for the linear actuation of the control member. In such a claw pole stepper motor the number of steps per revolution is limited by the geometry. Both the stator claws and the magnetic rotor poles must have certain minimum dimensions. In order to achieve a high resolution of the linear movement and a high actuation accuracy of the control member it is possible to use, for example, a motor with a large circumferential length, i.e. with a large diameter. However, such a motor has large motor dimensions, large mass moments of inertia and, consequently, a comparatively poor dynamic performance. In the known device a high resolution of the linear movement is achieved in that the threaded spindle has a screwthread with a comparatively fine pitch. It is often advantageous that in the case of faults, for example in the case of power failure, the control member automatically returns to its initial position. In the known device, where the threaded spindle has a thread with a comparatively fine pitch, this requires a very large return force, which is achieved in that the threaded spindle carries an electromagnet which, when energized, cooperates with an armature element and a rod and urges the rod into the threaded spindle against spring force. Such a construction is very intricate and expensive.